Multiple frequency magnetic field technique for differentiating between classes of metal objects

ABSTRACT

The invention relates to an A.C. metal detection apparatus employing a means for subjecting metal objects to oscillating magnetic fields of more than one frequency, and measuring circuits for detecting at each frequency the magnetic field due to the presence of the object and analyzing the magnetic fields to classify the objects on the basis of parameters such as size, shape, thickness, and surface to volume ratio.

United States Patent [151 3,686,564 Mallick, Jr. et a]. [4 1 Aug. 22,1972 [54] MULTIPLE FREQUENCY MAGNETIC 3,012,190 12/1961 Doll ..324/41FIELD TECHNIQUE FOR 3,500,373 3/1970 Minasy ..340/258 R DIFFERENTIATINGBETWE N CLASSES 0F METAL FOREIGN PATENTS OR APPLICATIONS 72] Inventors:George T. Mauick, Walter J. 798,999 11/1968 Canada ..324/4I Carr, Jr.;Robert C. Miller, all of I I Pittsburgh, p Przmary Exammer-Robert J.Corcoran A: --.H.H ,C.F.R dM.P.L h [73] Assignee: Westinghouse ElectricCorporation, tome), F enson am an ync PlttSbUfgIl, Pa. I [22] Flled:1970 The invention relates to an AC. metal detection ap- [21] Appl. No.:79,180 paratus employing a means for subjecting metal objects tooscillating magnetic fields of more than one 52 US. Cl. ..324/41 340/2580 frequency and measuring cimuits detecting at [51] Int. Cl. :Gfllr33/00 I each frequency the magnetic field due to the presence 58 FieldofSearch ..324/41, 40, 3; 340/258 R, of the Object and y ng the magneticfields to l I 253 C sify the objects on the bas1s of parameters such assize,

M I shape, thickness, and surface to volume ratio. [56] References C'ted18 Claims, 9 Drawing Figures UNITED STATES PATENTS 2,929,984 3/1960Puranen etal 324/3 I lo 54 FERR A UNETIC EXCESS /5e 52 I 32 METAL ALARMOSCILLATOR/ INDICATOR 'ND'CATOR U MlXER- OUT-OF-PHASEIJZ 48B 49 *1 4 92A lEB.1- a A OSCILfLQATOR A NPHASE b COMPONENT e L A retiree EXCITATIONCIRCUIT 3 o 22 |N PHAE 36 fg co lvrsgw'ole COMPONENT 4 4s T b MEASURINGo T455 CIRCUlTU C SIGNAL MEASURING 8 PROCESSING CIRCUITQQ COMPONENTMEASURING C lRCUITm-fl Patented Aug. 22, 1972 3 Sheets-Sheet 2 PatentedAug. 22, 1972 3 Sheets-Sheet 5 FIG.3.

All 6 3% 8 $0 Gmwzw IOOOOO I I0 IOOO IOOOO FREQUENCY (LOG SCALE)FERROMAGNETIC METAL OBJECT FIG. 4. L'NON-FERROMAGNETIC INSULATORNON-FERROMAGNETIC METAL OBJECT GUNS FREQUENCY Lul FIG.5B.

MULTIPLE FREQUENCY MAGNETIC FIELD TECHNIQUE FOR DIFFERENTIATING BETWEENCLASSES OF METAL OBJECTS ing oscillating magnetic fields are availableand have found use in applications such as mine detectors, prospectingdevices, and the like. These state-of-the-art devices, which usuallyemploy a measuring field oscil lating at a single frequency, detect thepresence of a metal object usually by measuring the power which theobject absorbs from the field, or by detecting the change in couplingbetween two coils produced by the object. The state-of-the-art devicesprovide little information concerning the geometry of the object, and donot have the capability, without being transported from point to point,of distinguishing between small metal objects close by and larger metalobjects further away.

The inability of current state-of-the-art metal detectors to clearlydiscriminate sufficiently between objects of varying size, shape andthickness renders them unsuitable for critical applications such as thedetection of concealed weapons.

SUMMARY OF THE INVENTION In this invention, oscillating magnetizingfields of various frequencies are generated through the use of one ormore magnetizing devices, shaped to produce a desired field distributionin the region of space through which the metal objects pass.

The magnetizing fields are provided most simply by one or moremagnetizing coils carrying A.C. current with a spectrum of frequencies.The geometry of the magnetizing coils is such as to produce a directionof magnetization having components of magnetizing field along theprincipal axes of interest in the metal object. For a concealed objectcarried through the field, such direction of magnetization may beachieved by a suitable distribution of this magnetizing field in space.For a concealed object fixed in space, a field along the principal axes,for various time intervals, is guaranteed by rotating the direction ofthe magnetizing field as a function of time.

' The spectrum of frequencies in each magnetizing coil includes at leasttwo frequencies, a high frequency, for which ideally the skin depth ofthe magnetic field of a specific class of metal objects of interest issmall compared with the thickness, and a low frequency for which thecalculated skin depth is large compared with the thickness.

A simple technique for investigating the induced field of a metal objectinvolves the use of detector coils positioned at various points inspace. As a special case, the magnetizing coils themselves may serve asdetector coils.

The voltage in the detector coils produced by the magnetizing fielditself is subtracted out by use of suitable electronics or electricalcircuits, so that only the voltage produced by the induced field of themetal object is measured. Electronic circuits are provided for measuringat each frequency the in-phase and out-ofphase component of the inducedvoltage with respect to the magnetizing current of that frequency.

The electronic measuring circuitry allows not only these components ofvoltage to be measured, but also provides for taking ratios of thevarious components, and in general allows comparison of the componentsafter various algebraic operations have been performed on them. Theoperation of taking ratios is particularly useful since it tends toeliminate dependence of the result on the magnitude of the appliedmagnetizing field.

Since the voltage measured in a detector coil depends upon the magneticflux from the metal object linking the detector coil, each voltagecomponent, at each frequency, depends upon the aforementioned materialand geometric parameters characterizing the metal object.

This dependence may be examined theoretically or empirically. A class ofobjects which are ferromagnetic with the general geometry of a hand gun,for example, will produce a set of responses in the measuring circuitsdifferent from other classes of objects. By selecting from all thepossible measurements, or combinations of measurements, those particularmeasurements which are most distinctive and which best categorize aclass of objects, a pattern for that class of objects is establishedfrom these measurements.

Electronic circuitry is used to recognize that pattern and to operate anindicator signal such as a light or an alarm when it occurs.

DESCRIPTION OF THE DRAWING The invention will become more readilyapparent from the following exemplary description in connection with theaccompanying drawing.

FIG. 1 is an electrical schematic illustration of an embodiment of theinvention;

FIGS. 2A, 2B, 2C and 2D are vector diagrams illustrating the operationof the invention disclosed in FIG.

FIG. 3 is a graphical illustration of curves plotting energy loss v.frequency for two classes of metal objects;

FIG. 4 is a graphical illustration of the out-of-phase component ofinduced voltage v. frequency of ferromagnetic and non-ferromagneticobjects subjected to magnetic fields by the embodiment of FIG. 1; and

FIGS. 5A and 5B are illustrations of alternate embodiments of theinvention as illustrated in FIG. 1.

DESCRIPTION OF THE PREFERRED EMBODIMENT Referring to FIG. 1 there isillustrated schematically a particular embodiment of the A.C.electromagnetic metal detector apparatus 10 comprising this invention.The A.C. electromagnetic metal detector of FIG. 1 is comprised of a coilconfiguration 20 including a magnetizing coil C and a detector coil C anexcitation circuit 30 operatively connected to the magnetizing coil Cand a signal measuring and processing circuit 40 operatively connectedto the detecting coil C The operation of the A.C. electromagnetic metaldetector apparatus 10 in accordance with the invention can be basicallydescribed as follows. A metal object is subjected to A.C. magneticfields of at least two frequencies, one high and one low, in response tothe excitation of the magnetizing coil C, by the excitation circuit 30.The irradiation of the metal object by these magnetic fields results inthe magnetization of the metal object. The detecting coil C is disposedrelative to the metal object to detect the magnetic field produced bythe magnetized metal object and to generate a signal corresponding tothis magnetic field. The ultimate in information to be derived from themagnetized object would necessitate the measurement of the magneticfield produced by the object from every point in space.

The signal measuring and processing circuit 40 derives the components ofthe signal generated by the detector coil C which are in-phase with thecurrent in the magnetizing coil C and the components which areout-of-phase with the current in the magnetizing coil C and processesthese component measurements to classify the metal object on the basisof parameters such as size, shape, thickness and surface-to-volumeratio.

The measurement of the in phase component at each frequency is a measureof the energy loss at each frequency attributable to the metal object.The measurement of the out-of-phase component indicates whether themetal object is ferromagnetic or non-ferromagnetic. For a ferromagneticobject at low frequency, the out-of-phase component tends to bedetermined by the induced magnetism of the object, and for anonferromagnetic metal object the out-of-phase component tends to bedetermined by the field of the induced eddy currents in the object. Fora non-ferromagnetic insulator, both the in-phase and out-of-phasecomponents tend to be very small.

Briefly then, the invention relates to the examination of magneticfields produced by metal objects which are subjected to AC. magneticfields of more than one frequency. While FIG. 1 illustrates the use ofboth a magnetizing coil C and a detector coil C the use of themagnetizing coil alone is sufficient to provide the magnetic fieldinformation required to classify the metal object. Furthermore, inasmuchas the essence of the invention concerns monitoring magnetic fields,other magnetic field responsive devices, such as a magnetometer, may beused in place of the coils.

In the specific embodiment of FIG. 1 coils C and C are arranged in atransformer-like configuration in which the magnetizing coil C functionsas the primary coil of the transformer by coupling energy to thedetector coil C which corresponds to the secondary coil of atransformer. Ideally in the operation of the transformer arrangement ofthe coil configuration 20 the magnetizing coil C causes an inducedvoltage to appear in the detector coil C which difiers in phase with thecurrent in the magnetizing coil C by 90. This vector relationship of anideal transformer (not exhibiting losses) is illustrated in FIG. 2Awherein the vector OA corresponds to the induced voltage. The presenceof a magnetically permeable core member or metal object will alter thisideal phase relationship resulting in rotation of the secondary voltagevector CA as illustrated in FIG. 2B. The change in vector relationshipbetween the induced secondary voltage and the current in the detectorcoil configuration 20 is utilized due to the presence of a metal objectby the AC. electromagnetic metal detector apparatus to classifymagnetically permeable objects passed within the active detecting area Adefined by coil C, on the basis of thickness, volume and ferromagneticor non-ferromagnetic characteristics. The signal measuring andprocessing circuit 40 provides the classification capability bymonitoring the characteristics of all magnetically permeable objectspassed within the field generated by magnetizing coil C and generating asignal to actuate the alarm circuit 52 in response to the presence of anobject of a specific class, such as a gun. The apparatus 10 will besubjected to extraneous magnetic influences existing in the surroundingoperational environment such as exist at airport facilities, and theseextraneous magnetic influences will result in a displacement of thesecondary induced voltage vector which is unrelated to the objects to bemonitored. The vector OA of FIG. 2C represents the displaced secondaryinduced voltage resulting from the extraneous magnetic fields as well asthe signal induced in the detector coil C by the excitation signalapplied to the magnetizing coil C The vector OC, which is out-of-phasewith the vector OA, corresponds to the influence of a phase shiftercircuit, such as a controlled mutual inductor 22, providing the null, orzero, operating conditions for the apparatus 10. Having accomplished thenull operating conditions for the apparatus 10, the insertion of amagnetically permeable object in the active field area A developed bycoil C results in a further displacement and change in length of thevector corresponding to the induced secondary voltage which isrepresented by vector OB of FIG. 2C. The resultant change in thesecondary induced voltage from vector 0A to vector OB is illustrated bya line A-B, which resultant may be resolved into component B, which isin-phase with the current in the magnetizing coil C and component B,which is out-of-phase with the current in the magnetizing coil C Thecomponent B is a measure of the electrical and magnetic energy loss(also referred to as eddy current loss or power absorbed by the object)produced by the magnetically permeable object and the component B, is anindication as to whether the object is ferromagnetic ornon-ferromagnetic. The measurement of these two parameters by the signalmeasuring and processing circuit 40 at more than one magnetizingexcitation frequency, as established by excitation circuit 30, providesthe information necessary to discriminate between a predetermined classof objects, such as guns, and other magnetically permeable objects.

Due to the current interest in detecting concealed firearms theoperation of the apparatus 10 will be concerned with discriminatingbetween magnetically permeable objects corresponding to small guns, andother miscellaneous magnetically permeable objects which are likely tobe present.

A plot of energy loss (power absorbed) versus magnetizing coil Cexcitation frequency for two classes of magnetically permeable objectsis depicted in curves A and B of FIG. 3. Curve A corresponds to objectscomposed of relatively thin metal elements such as keys, aerosol cans,radios, cameras, etc., while curve B corresponds to small guns. It isapparent from the curves of FIG. 3 that from the measurements of energyloss at, or between, two selected frequencies, information can bederived whereby the ratios or slopes AP and A P of the curves A and B,respectively, can be utilized to determine the classification of themagnetically penneable objects.

The magnetizing coil C excitation frequencies at which, or betweenwhich, the energy loss of the metal objects of the type corresponding tocurves A and B of FIG. 3 is to be measured are selected to provideratios of the measurements of the energy loss of the curves at theselected frequencies which will render the class of objects of interestclearly distinguishable from other objects likely to be monitored by theapparatus 10.

Assuming the detection of guns is of prime interest, the ratio A P ofcurve B defined between selected frequencies of approximately f (100hertz) and f 1,000 hertz) is clearly distinguishable from the ratio AP,, of curve A.

The selection of frequencies f,, and f to provide accurate gun detectiondiscrimination is based on the known condition that if an object issubjected to a sufficiently high frequency (f,,) to result in a skindepth in the object which is relatively small compared to the thickness(t) of the object, the power (P) absorbed by the object in energy lossvaries as f, while irradiation of the object by energy at a lowerfrequency (f,,), which results in the skin depth of the object beingrelatively large compared to the thickness (t), the power absorbed bythe object varies as 1". Thus, by making measurements of the powerabsorbed by an object at or between two frequencies, f and f it can bedetermined either that the object is relatively thin in comparison tothe skin depth if the following relationship is satisfied:

. fir $9 1. 9... it).

[DH/PL: (fa/f1)" (2) It is recognized that a gun is comprised of manythicknesses, thus the class of objects corresponding to guns fallsbetween the relationships l) and (2).

In the embodiment of the invention illustrated in FIG. 1 the oscillators32 and 34 of excitation circuit 30 generate the frequencies f,, and fwhich are combined simultaneously in the mixer-amplifier 36 andsubsequently applied to the magnetizing coil C The mutual inductor 22,as noted above, functions to offset the signal induced in the detectorcoil C by the excitation of the magnetizing coil C,, as well asoffsetting the influence of extraneous magnetic fields, to render a netzero input signal to the signal measuring and processing circuit 40 inthe absence of a metal object passing within the active detection areaA. The function of the mutual inductor 22 may be implemented by othermeans, or may be ignored completely if the signal induced in thedetector coil C by the magnetizing coil C can be tolerated.

Signal measuring and processing circuit 40 comprises a measuring circuit42 for measuring the out-of phase component of the signal induced indetector coil C at frequency J), an energy loss measuring circuit 44 formeasuring the energy loss (in-phase component) attributable to theobject at frequency f,,, and an energy loss measuring circuit 46 formeasuring the energy loss (in-phase component) attributable to theobject at frequency f,,. For the purpose of discussion the circuitselected to implement the operation of measuring circuits 42, 44 and 46of FIG. 1 is the phase lock amplifier of which the Princeton AppliedResearch (PAR) Model HRS is an example. The measuring circuits 42, 44and 46 will hereinafter be referred to as phase lock amplifiers 42, 44and 46. Each phase lock amplifier includes a first and second inputterminal a and b and an output terminal 0. The signal developed by thedetector coil C in response to the presence of a metal object in theactive area A of the coil configuration 20 is supplied to the inputs aof the phase lock amplifiers 42, 44 and 46 respectively, while areference signal R which is in phase with the current in magnetizingcoil C and contains signals (f +f,,,) is supplied to the input b of thephase lock amplifier circuits 42, 44 and 46. Each phase lock amplifierincludes input filter circuits and a reference signal phase controladjustment. The input filter circuits internally associated with thephase lock amplifiers 42 and 44 are designed to pass primarily the f,,component of the input signals, while the input filter circuitsinternally associated with the phase lock amplifier 46 are designed topass primarily the f component of the input signals.

Inasmuch as the component B as illustrated in FIGS. 2C and 2D, of theresultant vector displacement A-B is out-of-phase with the current and,therefore, as in a pure inductance, the phase lock amplifier 42 isadjusted to measure the component of f,, which is 90 out-of-phase withthe current in the magnetizing coil C Inasmuch as the energy losscomponent B is in-phase with the current in the magnetizing coil C as ina resistor, phase lock amplifiers 44 and 46 are adjusted to measure thein-phase components of f,, and f respectively. The phase lock amplifiers42, 44 and 46 generate DC output signals which are proportional to theaverage value of the detector coil input signals at the respectivefrequencies and respective phase relationships.

The DC output signal of the phase lock amplifier 42, is arbitrarilychosen to be positive if the metal object is ferromagnetic, as indicatedin FIG. 2C, and negative if the metal object is non-ferromagnetic asindicated in FIG. 2D. The determination of polarity of the output signalof the phase lock amplifier 42 is apparent from the plot of out-of-phasevoltage versus frequency in FIG. 4 for typical ferromagnetic metalobjects, non-ferromagnetic objects and non-ferromagnetic insulators.This information coupled with the determination of the energy lossattributed to the metal object at the selected frequencies f and fprovides three pieces of information which can be interpreted to definephysical characteristics of the detected object, such as thickness,which can be utilized to classify the detected objects according toclasses such as defined by curves A and B of FIG. 3. It is apparent thata fourth piece of information, i.e., the out-of-phase component at f canbe derived through the use of an additional measuring circuit 47 of thetype described above in reference to measuring circuits 42, 44 and 46.

Furthermore it is apparent that in-phase and out-ofphase measurements atmore than two frequencies can be implemented.

In the situation noted above where it is desired to discriminate betweena gun and other metal objects, the

information provided in the form of DC output signals can be processedthrough ratio circuit 45, comparator circuits 48A and 48B and logic ANDgate 49 to produce an actuation signal for the alarm circuit 52 when anobject having the characteristics of a gun is passed through the activesensing area A of the detector coil configuration 20.

Since most guns contain iron, the pattern of measurement made by thesignal measuring and processing circuit 40 corresponding to the class ofobjects comprising guns, includes:

l. a positive voltage output signal from phase lock amplifier 42, abovea selected threshold B set by comparator circuit 48B and defined in FIG.4 corresponding to the smallest hand gun to be detected. (Thiseliminates non-ferromagnetic and small ferromagnetic objects.)

2. a ratio measurement of low frequency energy loss to high frequencyenergy loss from ratio circuit 45 above a selected threshold C asdetermined by comparator circuit 48A. (This requirement tends todifferentiate between hand guns and other ferromagnetic metallicobjects.)

It follows therefore that the first measured characteristic of a gun isfulfilled if a positive DC output signal above the threshold B isdeveloped by the phase lock amplifier 42 resulting in the application ofa positive signal to the input 49a of the logic AND gate 49 bycomparator circuit 48B. The second measured characteristic required todevelop a positive signal at the input 49b of logic AND gate 49 isdetermined by the output of comparator circuit 48A. The DC. outputsignal of phase lock amplifier 44, corresponding to the energy loss at fis supplied to input 45a of the ratio circuit 45, and the DC. outputsignal of the phase lock amplifier 46, corresponding to the energy lossand f is supplied to the input 45b of the ratio circuit 45. The ratiocircuit 45 produces an output signal which is proportional to the ratioof the output signals of phase lock amplifiers 44 and 46. It has beendetermined that the ratio of high frequency energy loss to low frequencyenergy loss for guns, where f is approximately 1000 Hz and f isapproximately 100 Hz, is in the range of about 10 to l. The outputsignal of the ratio circuit is supplied as a first input signal tocomparator circuit 48A and a threshold signal C is supplied as a secondinput signal to comparator circuit 48A. The value of the thresholdsignal C corresponds to the metal object classification defined by curveB of FIG. 3 between frequencies f,, and f If the output signal from theratio circuit 45 is equal to or less than the threshold signal C, thecomparator circuit 48A generates a positive output signal indicating adetected metal object of the class defined by curve B. If this positivesignal coincides with a positive signal at the input terminal 49a, thecombination indicating a ferromagnetic object of a class comparable to agun, the logic AND gate 49 will transmit an actuating signal to an alarmcircuit 52. The processing of the in'phase and out-of-phase componentsthus described to provide alarm actuation is based primarily on objectthickness.

While the signal processing thus provided accounts for the majority ofsituations in which a gun is passed through the active detecting area,there exists two situations in which a gun would not result in an alarmactuating output signal from the logic AND gate 49. The first of thesesituations occurs if the gun is constructed of a non-ferromagneticmaterial, i.e., aluminum, in which case the DC. output signal of thephase lock amplifier 42 would be negative, and the second situationwhere the gun is accompanied by additional metallic objects producingenergy loss measurements by the phase lock amplifiers 44 and 46 whichresults in an output from ratio circuit 45 indicative of a metal objectoutside the classification for guns.

Either situation cannot be tolerated if optimum gun detection is to beaccomplished. Therefore the negative output from the phase lockamplifier 42 is connected to a non-ferromagnetic object detector 54, andan excess metal detector 56 is connected to the output of the phase lockamplifier 46. The information provided by indicators 54 and 56 alerts anoperator to the possibility of the passage of a gun to the activedetecting area thereby enabling the operator to take appropriate action.A typical relay voltmeter as marketed by Assembly Products Incorporatedcan be used to measure the magnitude of the signal developed by thephase lock amplifier 46 and provide an alarm actuation when themagnitude reaches a predetermined value indicative of excess metal. Itis noted that the output signal from phase lock amplifier 44 could alsobe used to indicate excess metal inasmuch as all metal exhibits losswhen subjected to magnetic fields. The amount of loss exhibited is indirect relationship to the amount of metal present.

While the embodiment of the invention depicted in FIG. 1, utilizing asingle coil configuration 20, illustrates the basic principle ofoperation of the invention, practical implementation of the invention toprovide optimum object detection and discrimination necessitates the useof coil configurations which provide either rotating field sensitivityor x, y and z axis sensitivity to the metal objects in order to minimizethe affect of the axis alignment of the metal object as it passesthrough the active detecting area A. This requirement is particularlycritical in detecting guns inasmuch as it is impossible to control theorientation of a gun as it passes through the active detecting area.Numerous combinations of coil configurations and associated electronicsmay be employed to generate the three mutually orthogonal magneticfields H H and H required to monitor the metal object and process theresults and information obtained.

Referring to FIGS. 5A and 58 there is illustrated typical implementationof coil configurations and circuitry for generating magnetic fields tocompletely enclose the path of travel of a metal object in order tomonitor the characteristics of the object in the manner disclosed abovewith respect to the operation of the embodiment of FIG. 1. Themulti-axis monitoring of the metal object provides optimum detection anddiscrimination regardless of orientation of the metal ob- 60 ject. Theembodiment illustrated in FIGS. 5A and 5B along the walk-throughstructure. The positioning of the coil configurations CC Cc and CCresults in a generation of three mutually orthogonal magnetic fieldscorresponding to the x, y and z axes respectively in response toexcitation circuits 30x, 30y and 302. The coil configurations CC,, CC,and CC correspond substantially to the coil configuration disclosedschematically in FIG. 1. One method of constructing the coilconfiguration involves the use of bifilar wire wherein one of theconductors corresponds to the magnetizing coil C of FIG. 1 and a secondconductor corresponds to the detector coil C of FIG. 1. The embodimentillustrated in FIG. 5A depicts each of the coil configurations ascomprising two segments A and B positioned in a parallel relationship onopposite surfaces of the walk-through structure 60. While the respectivex, y and z magnetic fields could be generated by using but one of thesegments of each of the coil configurations CC CC: and CC theutilization of the two segments provides for a more uniform magneticfield in each of three axes. While individual excitation circuits x, 30yand 301, and signal measuring and processing circuits x, 40y and 402 areconnected to the individual coil configurations to provide separatemeasurements of the metal object characteristics in each of the threeaxes, a single excitation circuit and a single signal measuring andprocessing circuit could be employed if the coil configurations areconnected in series. The circuitry employed in each of the circuits 30x,30y and 30:, and 40x, 40y and 402 corresponds substantially to thecircuitry utilized in the circuits 30 and 40 of FIG. 1. In theembodiment illustrated in FIG. 5A if an individual carrying a concealedweapon passes through the walk-through structure 60 one or more of therespective signal and processing circuits will generate an alarmactuating output signal depending on the orientation of the concealedweapon. Therefore the duplication of the AC. electromagnetic metaldetecting apparatus 10 of FIG. 1 for the x, y and z axes provides theoperating capability of discriminating between a gun and other metalobjects regardless of the orientation of the gun. The position of thecoil configuration CC CC and CC; successively along the walkthroughstructure 60 provides the isolation of each of the coil configurationswhich is required if the frequenciesf and f in each coil configurationare the same.

The embodiment illustrated in FIG. 5B depicts the coil configurationsCC,, CC, and CC as generating magnetic fields in the same passagewayvolume of the walk-through structure 60. In order to utilize thisforeshortened walk-through structure while providing discretemeasurements of the characteristics of the metal object at the x, y andz axes it is necessary to utilize three sufficiently distinct sets ofexcitation frequencies for the respective coil configurations to avoidinteraction between the measurements made. Once again the excitationcircuitry and the signal processing and measuring circuitry for the coilconfigurations of the embodiment illustrated in FIG. 5B correspondsubstantially to the circuitry utilized in the basic embodimentillustrated in FIG. 1.

We claim:

1. Apparatus for detecting preselected classes of metal objects,comprising,

an AC electromagnetic means including magnetizing means,

' a multi-frequency excitation circuit means operatively connected tosaid magnetizing means, said magnetizing means responding to excitationcurrent signals from said multi-frequency excitation circuit means bydeveloping at least first and second oscillating magnetic fields, saidfirst oscillating magnetic field being a low frequency oscillatingmagnetic field and said second oscillating magnetic field being a highfrequency oscillating magnetic field, said metal objects being subjectedto said oscillating magnetic fields,

detecting means for monitoring the induced magnetic fields developed bysaid metal objects being subjected to said low and high frequencyoscillating magnetic fields and developing output signals representativethereof, said signals including components at each frequency which arein-phase and out-of-phase with said excitation current signals, and

signal measuring and processing circuit means operatively connected tosaid detecting means, said signal measuring and processing meansincluding first circuit means responding to said in-phase components ateach frequency by generating first output signals indicative of energyloss attributable to said metal objects at each frequency second circuitmeans responding to said outof-phase components by generating secondoutput signals indicative of whether said metal objects areferromagnetic or nonferromagnetic, and third circuit means responsive tosaid first and second output signals for evaluating said signals andgenerating a third output signal indicative of the class of said metalobjects.

2. Apparatus as claimed in claim 1 wherein said low frequencyoscillating magnetic field being selected from a range of frequenciesfor which the skin depth of said induced magnetic field developed bysaid preselected class of metal objects is relatively large comparedwith the thickness, said high frequency oscillating magnetic field beingselected from a range of frequencies for which the skin depth of saidinduced magnetic field developed by said preselected class of metalobjects is relatively small compared to the thickness.

3. Apparatus as claimed in claim 2 wherein said magnetizing meansincludes a coil configuration having at least one magnetizing coil, andsaid detector means includes at least one detector coil for developingsaid output signals of said detector means as a function of the magneticflux linking said metal objects and said detector coil at each of thefrequencies of said low frequency magnetic field and said high frequencymagnetic field.

4. Apparatus as claimed in claim 1 further including circuit means forsubstantially eliminating from the signals developed by said detectormeans the effect of the excitation signals of said multi-frequencyexcitation circuit means.

5. Apparatus as claimed in claim 2 wherein said magnetizing meansincludes three coil configurations operatively connected to saidmulti-frequency excitation circuit means to generate three substantiallymutually orthogonal magnetic fields each comprised of a low and a highfrequency oscillating magnetic field,

said signal measuring and processing circuit means being operativelyconnected to said three coil configurations to provide detection anddiscrimination of metal objects passing through said magnetic fieldsregardless of orientation of said metal objects.

6. Apparatus as claimed in claim 5 wherein said three coilconfigurations are positioned about the path of travel of said metalobjects and are disposed in a successive manner such that said metalobjects are sequentially subjected to the magnetic fields of therespective coil configurations.

7. Apparatus as claimed in claim 5 wherein said three coilconfigurations are positioned about the path of travel of said metalobjects such that the metal objects are simultaneously subjected to themagnetic fields of the respective coil configurations.

8. Apparatus as claimed in claim 6 wherein the low and high frequencymagnetic fields of each coil configuration are the same.

9. Apparatus as claimed in claim 7 wherein the low and high frequencymagnetic fields of each coil configuration are different.

10. Apparatus as claimed in claim 1 wherein said out-of-phase componentsare approximately 90 outof-phase with said excitation signals.

11. Apparatus as claimed in claim 1 wherein said oscillating magneticfields are developed simultaneously.

12. Apparatus as claimed in claim 1 wherein said first circuit meansdetermines the relative magnitudes of the signal componentscorresponding to energy loss at said low and high frequency, said firstoutput signals representing the relationship of these magnitudes, saidthird circuit means responding to said first and second output signalsfor comparing said signals to predetermined values, said third outputsignal being generated if said first and second output signals establisha predetermined relationship with said predetermined values.

13. Apparatus for detecting preselected classes of metal objects,comprising,

an AC. electromagnetic means including magnetizing means,

a multi-frequency excitation circuit means operatively connected to saidmagnetizing means, said magnetizing means responding to excitationcurrent signals from said multi-frequency excitation circuit means bydeveloping oscillating magnetic fields, said metal objects beingsubjected to said oscillating magnetic fields, said oscillating magneticfields including at least one low frequency magnetic field and at leastone high frequency magnetic field,

detecting means for monitoring the induced magnetic field developed bysaid metal objects being subjected to said oscillating magnetic fieldsand developing output signals corresponding to said low and highfrequency magnetic fields, and

signal measuring and processing circuit means including first circuitmeans responding to said output signals of said detecting means producedby said low frequency oscillating magnetic field by developing a firstoutput signal which is out-ofphase with said excitation current signals,second and third circuit means for developing second and third outputsignals indicative of components of said ou ut si als of said detectinmeans 'ch are pro iicedg said low frequency and saic i igh frequencyoscillating magnetic fields respectively, said second and third outputsignals being in phase with said excitation current signals, said firstoutput signal being an indication of whether said metal objects areferromagnetic or nonferromagnetic, said second and third output signalsbeing a measurement of the energy loss attributable to said metalobjects at said low frequency and high frequency oscillating magneticfields respectively, fourth circuit means responsive to said second andthird output signals for determining the relative magnitudes of saidsecond and third output signals and developing a fourth output signalindicative of this relationship, and object classification circuit meansresponsive to said first output signal and said fourth output signal forgenerating an object classification output signal indicative of aspecific class of metal objects.

14. Apparatus as claimed in claim 13 wherein said specified class ofmetal objects is small arms, such as pistols, the frequency of said lowfrequency magnetic field being approximately hertz, and the frequency ofsaid high frequency magnetic field being approximately 1,000 hertz.

15. Apparatus as claimed in claim 13 wherein said fourth output signalis indicative of a ratio between the energy loss at the high frequencyoscillating magnetic field and the energy loss at the low frequencyoscillating magnetic field.

16. Apparatus as claimed in claim 13 wherein said second and thirdcircuit means include a second and third phase lock amplifier circuitrespectively, each having a first and second input and an output, saidoutput signals from said detector means being supplied to said firstinputs and a reference signal comprised of the low frequency and highfrequency signals developed by said multi-frequency excitation circuitbeing supplied to said second inputs, said second phase lock amplifiercircuit developing said second output signal, said third phase lockamplifier circuit developing said third output signal, said second andthird output signals corresponding to the energy loss attributable tosaid metal objects at said low and high frequencies respectively.

17. Apparatus as claimed in claim 13 wherein said first circuit meansfurther includes a first phase lock amplifier circuit having a first andsecond input and an output, said output signals from said detector meansbeing supplied to said first input and said reference signal beingsupplied to said second input, said first phase lock amplifier circuitdeveloping said first output signal.

18. Apparatus as claimed in claim 15 wherein said ratio for a class ofmetal objects corresponding to small arms is approximately 10:1.

1. Apparatus for detecting preselected classes of metal objects,comprising, an A.C. electromagnetic means including magnetizing means, amulti-frequency excitation circuit means operatively connected to saidmagnetizing means, said magnetizing means responding to excitationcurrent signals from said multi-frequency excitation circuit means bydeveloping at least first and second oscillating magnetic fields, saidfirst oscillating magnetic field being a low frequency oscillatingmagnetic field and said second oscillating magnetic field being a highfrequency oscillating magnetic field, said metal objects being subjectedto said oscillating magnetic fields, detecting means for monitoring theinduced magnetic fields developed by said metal objects being subjectedto said low and high frequency oscillating magnetic fields anddeveloping output signals representative thereof, said signals includingcomponents at each frequency which are in-phase and out-ofphase withsaid excitation current signals, and signal measuring and processingcircuit means operatively connected to said detecting means, said signalmeasuring and processing means including first circuit means respondingto said in-phase components at each frequency by generating first outputsignals indicative of energy loss attributable to said metal objects ateach frequency, second circuit means responding to said out-of-phasecomponents by generating second output signals indicative of whethersaid metal objects are ferromagnetic or nonferromagnetic, and thirdcircuit means responsive to said first and second output signals forevaluating said signals and generating a third output signal indicativeof the class of said metal objects.
 2. Apparatus as claimed in claim 1wherein said low frequency oscillating magnetic field being selectedfrom a range of frequencies for which the skin depth of said inducedmagnetic field developed by said preselected class of metal objects isrelatively large compared with the thickness, said high frequencyoscillating magnetic field being selected from a range of frequenciesfor which the skin depth of said induced magnetic field developed bysaid preselected class of metal objects is relatively small compared tothe thickness.
 3. Apparatus as claimed in claim 2 wherein saidmagnetizing means includes a coil configuration having at least onemagnetizing coil, and said detector means includes at least one detectorcoil for developing said output signals of said detector means as afunction of the magnetic flux linking said metal objects and saiddetector coil at each of the frequencies of said low frequency magneticfield and said high frequency magnetic field.
 4. Apparatus as claimed inclaim 1 further including circuit means for substantially eliminatingfrom the signals developed by said detector means the effect of theexcitation signals of said multi-frequency excitation circuit means. 5.Apparatus as claimed in claim 2 wherein said magnetizing means includesthree coil configurations operatively connected to said multi-frequencyexcitation circuit means to generate three substantially mutuallyorthogonal magnetic fields each comprised of a low and a high frequencyoscillating magnetic field, said signal measuring and processing circuitmeans being operatively connected to said three coil configurations toprovide detection and discrimination of metal objects passing throughsaid magnetic fields regardless of orientation of said metal objects. 6.Apparatus as claimed in claim 5 wherein said three coil configurationsare positioned about the path of travel of said metal objects and aredisposed in a successive manner such that said metal objects aresequentially subjected to the magnetic fields of the respective coilconfigurations.
 7. Apparatus as claimed in claim 5 wherein said threecoil configurations are positioned about the path of travel of saidmetal objects such that the metaL objects are simultaneously subjectedto the magnetic fields of the respective coil configurations. 8.Apparatus as claimed in claim 6 wherein the low and high frequencymagnetic fields of each coil configuration are the same.
 9. Apparatus asclaimed in claim 7 wherein the low and high frequency magnetic fields ofeach coil configuration are different.
 10. Apparatus as claimed in claim1 wherein said out-of-phase components are approximately 90*out-of-phase with said excitation signals.
 11. Apparatus as claimed inclaim 1 wherein said oscillating magnetic fields are developedsimultaneously.
 12. Apparatus as claimed in claim 1 wherein said firstcircuit means determines the relative magnitudes of the signalcomponents corresponding to energy loss at said low and high frequency,said first output signals representing the relationship of thesemagnitudes, said third circuit means responding to said first and secondoutput signals for comparing said signals to predetermined values, saidthird output signal being generated if said first and second outputsignals establish a predetermined relationship with said predeterminedvalues.
 13. Apparatus for detecting preselected classes of metalobjects, comprising, an A.C. electromagnetic means including magnetizingmeans, a multi-frequency excitation circuit means operatively connectedto said magnetizing means, said magnetizing means responding toexcitation current signals from said multi-frequency excitation circuitmeans by developing oscillating magnetic fields, said metal objectsbeing subjected to said oscillating magnetic fields, said oscillatingmagnetic fields including at least one low frequency magnetic field andat least one high frequency magnetic field, detecting means formonitoring the induced magnetic field developed by said metal objectsbeing subjected to said oscillating magnetic fields and developingoutput signals corresponding to said low and high frequency magneticfields, and signal measuring and processing circuit means includingfirst circuit means responding to said output signals of said detectingmeans produced by said low frequency oscillating magnetic field bydeveloping a first output signal which is out-of-phase with saidexcitation current signals, second and third circuit means fordeveloping second and third output signals indicative of components ofsaid output signals of said detecting means which are produced by saidlow frequency and said high frequency oscillating magnetic fieldsrespectively, said second and third output signals being in phase withsaid excitation current signals, said first output signal being anindication of whether said metal objects are ferromagnetic ornonferromagnetic, said second and third output signals being ameasurement of the energy loss attributable to said metal objects atsaid low frequency and high frequency oscillating magnetic fieldsrespectively, fourth circuit means responsive to said second and thirdoutput signals for determining the relative magnitudes of said secondand third output signals and developing a fourth output signalindicative of this relationship, and object classification circuit meansresponsive to said first output signal and said fourth output signal forgenerating an object classification output signal indicative of aspecific class of metal objects.
 14. Apparatus as claimed in claim 13wherein said specified class of metal objects is small arms, such aspistols, the frequency of said low frequency magnetic field beingapproximately 100 hertz, and the frequency of said high frequencymagnetic field being approximately 1,000 hertz.
 15. Apparatus as claimedin claim 13 wherein said fourth output signal is indicative of a ratiobetween the energy loss at the high frequency oscillating magnetic fieldand the energy loss at the low frequency oscillating magnetic field. 16.Apparatus as claimed in claim 13 wherein said second and third circuitmeans include a second and tHird phase lock amplifier circuitrespectively, each having a first and second input and an output, saidoutput signals from said detector means being supplied to said firstinputs and a reference signal comprised of the low frequency and highfrequency signals developed by said multi-frequency excitation circuitbeing supplied to said second inputs, said second phase lock amplifiercircuit developing said second output signal, said third phase lockamplifier circuit developing said third output signal, said second andthird output signals corresponding to the energy loss attributable tosaid metal objects at said low and high frequencies respectively. 17.Apparatus as claimed in claim 13 wherein said first circuit meansfurther includes a first phase lock amplifier circuit having a first andsecond input and an output, said output signals from said detector meansbeing supplied to said first input and said reference signal beingsupplied to said second input, said first phase lock amplifier circuitdeveloping said first output signal.
 18. Apparatus as claimed in claim15 wherein said ratio for a class of metal objects corresponding tosmall arms is approximately 10:1.